Project Summary Evidence from our lab1 and others2,3,4 finds that an individual's sensitivity to alcohol in emerging adulthood predicts future alcohol-related problems. Although sensitivity has been hypothesized to relate to alcohol tolerance, few studies have adequately evaluated that relationship. In addition, our prior research has identified the prefrontal cortex (PFC) and limbic-hypothalamic-pituitary-adrenal (LHPA) stress axis, which interact with the brain's reward circuit, as a potential sex-sensitive functional brain network associated with a person's low level of response (LR) to alcohol. Until now, functional magnetic resonance imaging (fMRI)-based associations with LR have analyzed brain regional alcohol effects in a segregated manner and did not consider the functional connectivity (FC) between the PFC-LHPA network and sex differences in these brain circuits. The goals of this proposal are to conduct a series of post-hoc secondary analyses, two pilot feasibility trials, and a full human laboratory study that combines neuroendocrine and ethanol-fMRI approaches to probe the PFC-LHPA network through a lens focused on LR, alcohol's stimulating effects, the development of acute tolerance, and potential sex differences. We hypothesize that low LR to alcohol will be correlated with decreased FC between the PFC-LHPA network and that disrupted FC will be greater in women than men. To test this hypothesis, in the R21 phase we will reanalyze fMRI data from an existing data set wherein low- and high-LR women and men completed an alcohol challenge followed by an ethanol/placebo-fMRI emotional processing task. We will also explore potential sex differences and the relations among LR to alcohol with: 1) acute tolerance measured previously in the lab, and 2) the subsequent development of chronic tolerance measured longitudinally in two unique existing data sets. Finally, we will conduct two pilot feasibility studies in 12 low- and high-LR women and men that will serve as Go / No-Go decision points for the R33 phase: 1) examine subjective and objective alcohol sensitivity measures and acute tolerance to a higher dose of alcohol; and 2) examine alcohol's effects on brain activation and connectivity patterns following emotional- and stress-processing fMRI tasks that stimulate different components of the PFC-LHPA network. In the R33 phase, we will conduct a new set of experiments combining a higher-dose ethanol neuroendocrine challenge piloted in the R21 phase followed by the ethanol/placebo-fMRI procedure piloted previously in 60 young, moderate drinking, non-alcohol dependent women and men. That experiment will allow us to test whether low- (N = 30) and high-LR (N = 30) women (50%) and men differ in their stress hormone response to a binge-like drinking episode, and how those alcohol sensitivity and stress hormone responses correlate with functional connectivity in the PFC-LHPA network. Data will also be gathered to evaluate alcohol's stimulating, hedonic, ataxic effects and acute tolerance as measured by changes in endocrine and PFC-LHPA measures at rising versus falling breath alcohol concentrations between low- and high-LR subjects.